I was having the same problem and this fixed it. Hopefully this will work for others.

NOTE: This will not work if you don't have your sync settings correct.

Just follow these steps:
Press your device's < Home > button, then press < Menu >. Select Settings. Select Applications, then Manage applications. Select Gmail Storage.
Note: If you don't see 'Gmail Storage,' press < Menu > again, then select Filter >All. Select Clear data. Answer OK when prompted. Return to the list of applications by pressing the < Back > button on your device. Select Gmail and then select Clear data. Answer OK when prompted. Then do a restart. To restart your device, press and hold the < Power > or < End call > button, then select Power off. Press the < Power > or < End call > button to power on. Your device will re-sync with the server after restarting (this may take several minutes).